Drs. Dennis Tokaryk and Stephen Ross from the University of New Brunswick and their collaborators at The Ohio State University have used the Canadian Light Source (CLS) at the University of Saskatchewan to provide a purely experimental confirmation of the existence of a phenomenon called quantum monodromy. Their findings were published recently as the cover article in the academic journal Physical Chemistry Chemical Physics. Their article has since been identified as a “HOT PCCP article” by the journal.
Monodromy (Greek for “once around”) is involved in multiple areas of physics, from the expansion of the early universe to string theory and even to black holes. Theoretical models and previous experimental evidence had pointed to the presence of quantum monodromy in some molecules, including water.
To understand monodromy it helps to first think of a system that has two important properties that can take different values. (A simple example of a pair of properties could be temperature and pressure - although there is no known monodromy involving these particular two properties.) The next step is to think of sequentially changing those two properties, perhaps first increasing both, then continuing by further increasing just one, and so on. One could do this in such a way that all the increases are cancelled by subsequent decreases in each property. At that point the system should have returned to its initial state. Ross says monodromy is when a system has loops in its parameters where, when you return to the start, the properties are different from how they began. “This is a surprising result,” says Ross.
In the case of a chain molecule, such as NCNCS, we can think of its bending vibration and its (axial) rotation as the two properties. Each can be increased or decreased. Tokaryk and Ross and collaborators showed that there are indeed such special paths in this molecule. That is, paths where what was considered a rotation when the path is started, had become a mixture of rotation and vibration, at the end of the path. “The consequence is that in molecules such as NCNCS, the meaning of rotation and vibration is ambiguous!” says Ross. (Going back to the completely hypothetical example of temperature and pressure, this would be equivalent to saying that the meaning of temperature and pressure is ambiguous, that they are somehow mixed together.)
Dr. Tokaryk says the bright light of the CLS was critical in identifying this effect. Over four three-week visits to the CLS synchrotron, he and the team gathered all the data necessary to definitively show the quantum monodromy phenomenon in NCNCS. “The collaboration required innovative work starting with creating the molecule in-house, probing it with radiation from the CLS synchrotron, processing the data, refining theoretical models, and finally understanding the implications of the results,” say Tokaryk and Ross. “This work took us a decade to complete.”
Now that the purely experimental proof of quantum monodromy is complete, Tokaryk and Ross are looking forward to seeing how other researchers make use of this concept in other systems.
Tokaryk, Dennis W., Stephen C. Ross, Manfred Winnewisser, Brenda P. Winnewisser, Frank C. De Lucia, and Brant E. Billinghurst. "Quantum monodromy in NCNCS–direct experimental confirmation." Physical Chemistry Chemical Physics (2023). https://doi.org/10.1039/D3CP01420G
Canadian Light Source